Mixed Organic Counterion Strategy Modulates the Self-Assembly of Polyoxometalate Hybrids into Toroids and Affects Their Photochromic and Photocatalytic Properties

A series of five mixed-counterion polyoxometalate (POM) hybrids 1-5 have been developed starting from the [SiW12O40]4- cluster and two different aryl sulfonium counterions (ASCIs), (4-hydroxyphenyl)dimethylsulfonium (HPDS) and (4-formylphenyl)dimethylsulfonium (FPDS), bearing different functional groups -OH and -CHO, respectively. The HPDS/FPDS counterion ratio in hybrids 1-5 varied as 4:0 (1), 3:1 (2), 2:2 (3), 1:3 (4), and 0:4 (5), and it resulted in a morphological transition of the self-assembly of these hybrids. Hybrid 1 exhibited spherical self-assembly, while hybrid 5 exhibited large toroidal self-assembly predominantly. The hybrids 2-4 gave a mixture of spherical, dimpled spherical, and toroidal self-assemblies in varying amounts, indicating a gradual transition of self-assembly from spherical to toroidal as the percentage of FPDS increases in the system. A plausible explanation for the differences in the self-assembly properties of these hybrids has been proposed based on the differences in the H-bonding interactions of the two counterions with the dispersion medium. The HPDS and FPDS counterions exhibited different HOMO-LUMO gaps, and their varying percentages in hybrids 1-5 led to differences in optical band gaps and photochromic activities. Hybrid 2 showed the lowest band gap value and the best photochromic activity (t1/2 = 0.72 min) in the series. The influence of the mixed-counterion strategy in catalysis has been demonstrated by evaluating the photocatalytic activity of these hybrids toward rhodamine B dye degradation. The hybrid 4, exhibiting stable toroidal self-assembly, showed the best catalytic activity in the series, probably due to a combination of reasons such as the enhanced surface area due to toroid formation and also the presence of H-bonding -OH functionality on the counterion. Therefore, the present study suggests a new method for altering the self-assembly and the materials and catalytic properties of POM hybrids through a mixed-counterion strategy.